Sustained integrin ligation involves extracellular free sulfhydryls and enzymatically catalyzed disulfide exchange

Platelets: Physiology and Biochemistry

This article represents a republication of an article originally published in STH in 2005. This republication is to help celebrate 50 years of publishing for STH. The original abstract follows.Platelets are specialized blood cells that play central roles in physiologic and pathologic processes of hemostasis, inflammation, tumor metastasis, wound healing, and host defense. Activation of platelets is crucial for platelet function that includes a complex interplay of adhesion and signaling molecules. This article gives an overview of the activation processes involved in primary and secondary hemostasis, for example, platelet adhesion, platelet secretion, platelet aggregation, microvesicle formation, and clot retraction/stabilization. In addition, activated platelets are predominantly involved in cross-talk to other blood and vascular cells. Stimulated "sticky" platelets enable recruitment of leukocytes at sites of vascular injury under high shear conditions. Platelet-derived microparticles as well as soluble adhesion molecules, sP-selectin and sCD40L, shed from the surface of activated platelets, are capable of activating, in turn, leukocytes and endothelial cells. This article focuses further on the new view of receptor-mediated thrombin generation of human platelets, necessary for the formation of a stable platelet-fibrin clot during secondary hemostasis. Finally, special emphasis is placed on important stimulatory and inhibitory signaling pathways that modulate platelet function.

Protein disulfide isomerase A1 regulates fenestration dynamics in primary mouse liver sinusoidal endothelial cells (LSECs)

Protein disulfide isomerases (PDIs) are involved in many intracellular and extracellular processes, including cell adhesion and cytoskeletal reorganisation, but their contribution to the regulation of fenestrations in liver sinusoidal endothelial cells (LSECs) remains unknown. Given that fenestrations are supported on a cytoskeleton scaffold, this study aimed to investigate whether endothelial PDIs regulate fenestration dynamics in primary mouse LSECs. PDIA3 and PDIA1 were found to be the most abundant among PDI isoforms in LSECs. Taking advantage of atomic force microscopy, the effects of PDIA1 or PDIA3 inhibition on the fenestrations in LSECs were investigated using a classic PDIA1 inhibitor (bepristat) and novel aromatic N-sulfonamides of aziridine-2-carboxylic acid derivatives as PDIA1 (C-3389) or PDIA3 (C-3399) inhibitors. The effect of PDIA1 inhibition on liver perfusion was studied in vivo using dynamic contrast-enhanced magnetic resonance imaging. Additionally, PDIA1 inhibitors were examined in vitro in LSECs for effects on adhesion, cytoskeleton organisation, bioenergetics, and viability. Inhibition of PDIA1 with bepristat or C-3389 significantly reduced the number of fenestrations in LSECs, while inhibition of PDIA3 with C-3399 had no effect. Moreover, the blocking of free thiols by the cell-penetrating N-ethylmaleimide, but not by the non-cell-penetrating 4-chloromercuribenzenesulfonate, resulted in LSEC defenestration. Inhibition of PDIA1 did not affect LSEC adhesion, viability, and bioenergetics, nor did it induce a clear-cut rearrangement of the cytoskeleton. However, PDIA1-dependent defenestration was reversed by cytochalasin B, a known fenestration stimulator, pointing to the preserved ability of LSECs to form new pores. Importantly, systemic inhibition of PDIA1 in vivo affected intra-parenchymal uptake of contrast agent in mice consistent with LSEC defenestration. These results revealed the role of intracellular PDIA1 in the regulation of fenestration dynamics in LSECs, and in maintaining hepatic sinusoid homeostasis.

Molecular basis of clot retraction and its role in wound healing

Clot retraction is important for the prevention of bleeding, in the manifestations of thrombosis and for tissue repair. The molecular mechanisms behind clot formation are complex. Platelet involvement begins with adhesion at sites of vessel injury followed by platelet aggregation, thrombin generation and fibrin production. Other blood cells incorporate into a fibrin mesh that is consolidated by FXIIIa-mediated crosslinking and platelet contractile activity. The latter results in the asymmetric redistribution of erythrocytes into a tighter central mass providing the clot with stability and resistance to fibrinolysis. Integrin αIIbβ3 on platelets is the key player in these events, bridging fibrin and the platelet cytoskeleton. Glycoprotein VI participates in thrombus formation but not in the retraction. Rheological and environmental factors influence clot construction with retraction driven by the platelet cytoskeleton with actomyosin acting as the motor. Activated platelets provide procoagulant activity stimulating thrombin generation together with the release of a plethora of biologically active proteins and substances from storage pools; many form chemotactic gradients within the fibrin or the underlying matrix. Also released are newly synthesized metabolites and lipid-rich vesicles that circulate within the vasculature and mimic platelet functions. Platelets and their released elements play key roles in wound healing. This includes promoting stem cell and mesenchymal stromal cell recruitment, fibroblast and endothelial cell migration, angiogenesis and matrix formation. These properties have led to the use of autologous clots in therapies designed to accelerate tissue repair while offering the potential for genetic manipulation in both inherited and acquired diseases.

High-throughput microfluidic blood testing to phenotype genetically linked platelet disorders: an aid to diagnosis

Linking the genetic background of patients with bleeding diathesis and altered platelet function remains challenging. We aimed to assess how a multiparameter microspot-based measurement of thrombus formation under flow can help identify patients with a platelet bleeding disorder. For this purpose, we studied 16 patients presenting with bleeding and/or albinism and suspected platelet dysfunction and 15 relatives. Genotyping of patients revealed a novel biallelic pathogenic variant in RASGRP2 (splice site c.240-1G>A), abrogating CalDAG-GEFI expression, compound heterozygosity (c.537del, c.571A>T) in P2RY12, affecting P2Y12 signaling, and heterozygous variants of unknown significance in the P2RY12 and HPS3 genes. Other patients were confirmed to have Hermansky-Pudlak syndrome type 1 or 3. In 5 patients, no genetic variant was found. Platelet functions were assessed via routine laboratory measurements. Blood samples from all subjects and day controls were screened for blood cell counts and microfluidic outcomes on 6 surfaces (48 parameters) in comparison with those of a reference cohort of healthy subjects. Differential analysis of the microfluidic data showed that the key parameters of thrombus formation were compromised in the 16 index patients. Principal component analysis revealed separate clusters of patients vs heterozygous family members and control subjects. Clusters were further segregated based on inclusion of hematologic values and laboratory measurements. Subject ranking indicated an overall impairment in thrombus formation in patients carrying a (likely) pathogenic variant of the genes but not in asymptomatic relatives. Taken together, our results indicate the advantages of testing for multiparametric thrombus formation in this patient population.

Current concepts and novel targets for antiplatelet therapy

Platelets have a crucial role in haemostasis and atherothrombosis. Pharmacological control of platelet hyper-reactivity has become a cornerstone in the prevention of thrombo-ischaemic complications in atherosclerotic diseases. Current antiplatelet therapies substantially improve clinical outcomes in patients with coronary artery disease, but at the cost of increased risk of bleeding. Beyond their role in thrombosis, platelets are known to regulate inflammatory (thrombo-inflammatory) and microcirculatory pathways. Therefore, controlling platelet hyper-reactivity might have implications for both tissue inflammation (myocardial ischaemia) and vascular inflammation (vulnerable plaque formation) to prevent atherosclerosis. In this Review, we summarize the pathophysiological role of platelets in acute myocardial ischaemia, vascular inflammation and atherosclerotic progression. Furthermore, we highlight current clinical concepts of antiplatelet therapy that have contributed to improving patient care and have facilitated more individualized therapy. Finally, we discuss novel therapeutic targets and compounds for antiplatelet therapy that are currently in preclinical development, some of which have a more favourable safety profile than currently approved drugs with regard to bleeding risk. These novel antiplatelet targets might offer new strategies to treat cardiovascular disease.

Development of an in-vitro model for extracorporeal blood pumps to study the effects of artificial pulsatility on human blood

Introduction

The application of extracorporeal circulation (ECC) systems is known to be associated with several implications regarding hemolysis, inflammation, and coagulation. In the last years, systems with pulsatile blood flow are increasingly used with the intention to improve hemodynamics in reperfusion. However, their implications on the aforementioned aspects remain largely unknown. To investigate the effects of pulsatility, this ex-vivo study was initiated.

Methods

Test circuits (primed with human whole blood) were set up in accordance with the recommendations of international standards for in-vitro evaluation of new components and systems of ECC. Diagonal pumps were either set up with non-pulsatile (n = 5, NPG) or pulsatile (n = 5, PG) pump settings and evaluated for 6 h. All analyses were conducted with human whole blood. Blood samples were repeatedly drawn from the test circuits and analyzed regarding free hemoglobin, interleukin 8 (IL-8), platelet aggregation and acquired von Willebrand syndrome (AVWS).

Results

After 1 h of circulation, a significant coagulation impairment (impaired platelet function and AVWS) was observed in both groups. After 6 h of circulation, increased IL-8 concentrations were measured in both groups (NPG: 0.05 ± 0.03 pg./mL, PG: 0.03 ± 0.01 pg./mL, p = 0.48). Pulsatile pump flow resulted in significantly increased hemolysis after 6 h of circulation (NPG: 37.3 ± 12.4 mg/100 L; PG: 59.6 ± 14.5 mg/100 L; p < 0.05).

Conclusion

Our results indicate that the coagulative impairment takes place in the early phase of ECC. Pulsatility did not affect the occurrence of AVWS ex-vivo. Prolonged durations of pulsatile pump flow led to increased hemolysis and therefore, its prolonged use should be employed cautiously in clinical practice with appropriate monitoring.

A targeted activatable NIR-II nanoprobe for positive visualization of anastomotic thrombosis and sensitive identification of fresh fibrinolytic thrombus

Anastomotic thrombosis prevalently causes anastomosis failure, accompanied with ischemia and necrosis, the early diagnosis of which is restricted by inherent shortcomings of traditional imaging techniques in clinic and lack of appropriate prodromal biomarkers for thrombosis initiation. Herein, a fresh thrombus-specific molecular event, protein disulfide isomerase (PDI) is innovatively chosen as the activating factor, and a thrombosis targeting and PDI-responsive turn-on near infrared II (NIR-II) fluorescence nanoprobe is firstly developed. The supramolecular complex-based nanoprobe IR806-PDA@BSA-CREKA is fabricated by assembling NIR-II emitting cyanine derivative IR806-PDA with bovine serum albumin (BSA), which could ameliorate the stability and pharmacokinetics of the nanoprobe, addressing the contradiction in the balance of brightness and biocompatibility. The NIR-II-off nanoprobe exhibits robust turn-on NIR-II fluorescence upon PDI-specific activation, in vitro and in vivo. Of note, the constructed nanoprobe demonstrates superior photophysical stability, efficient fibrin targeting peptide-derived thrombosis binding and a maximum signal-to-background ratio (SBR) of 9.30 for anastomotic thrombosis in NIR-II fluorescent imaging. In conclusion, the exploited strategy enables positive visualized diagnosis for anastomotic thrombosis and dynamic monitoring for thrombolysis of fresh fibrinolytic thrombus, potentially contributes a novel strategy for guiding the therapeutic selection between thrombolysis and thrombectomy for thrombosis treatment in clinic.

Protein Disulfide Isomerase and Extracellular Adherence Protein Cooperatively Potentiate Staphylococcal Invasion into Endothelial Cells

Invasion of host cells is an important feature of Staphylococcus aureus. The main internalization pathway involves binding of the bacteria to host cells, e.g., endothelial cells, via a fibronectin (Fn) bridge between S. aureus Fn binding proteins and α5β1-integrin, followed by phagocytosis. The secreted extracellular adherence protein (Eap) has been shown to promote this cellular uptake pathway of not only S. aureus, but also of bacteria otherwise poorly taken up by host cells, such as Staphylococcus carnosus. The exact mechanisms are still unknown. Previously, we demonstrated that Eap induces platelet activation by stimulation of the protein disulfide isomerase (PDI), a catalyst of thiol-disulfide exchange reactions. Here, we show that Eap promotes PDI activity on the surface of endothelial cells, and that this contributes critically to Eap-driven staphylococcal invasion. PDI-stimulated β1-integrin activation followed by increased Fn binding to host cells likely accounts for the Eap-enhanced uptake of S. aureus into non-professional phagocytes. Additionally, Eap supports the binding of S. carnosus to Fn-α5β1 integrin, thereby allowing its uptake into endothelial cells. To our knowledge, this is the first demonstration that PDI is crucial for the uptake of bacteria into host cells. We describe a hitherto unknown function of Eap-the promotion of an enzymatic activity with subsequent enhancement of bacterial uptake-and thus broaden mechanistic insights into its importance as a driver of bacterial pathogenicity. IMPORTANCE Staphylococcus aureus can invade and persist in non-professional phagocytes, thereby escaping host defense mechanisms and antibiotic treatment. The intracellular lifestyle of S. aureus contributes to the development of infection, e.g., in infective endocarditis or chronic osteomyelitis. The extracellular adherence protein secreted by S. aureus promotes its own internalization as well as that of bacteria that are otherwise poorly taken up by host cells, such as Staphylococcus carnosus. In our study, we demonstrate that staphylococcal uptake by endothelial cells requires catalytic disulfide exchange activity by the cell-surface protein disulfide isomerase, and that this critical enzymatic function is enhanced by Eap. The therapeutic application of PDI inhibitors has previously been investigated in the context of thrombosis and hypercoagulability. Our results add another intriguing possibility: therapeutically targeting PDI, i.e., as a candidate approach to modulate the initiation and/or course of S. aureus infectious diseases.

Inhibition of Cell Motility by Cell-Penetrating Dynamic Covalent Cascade Exchangers: Integrins Participate in Thiol-Mediated Uptake

Integrins are cell surface proteins responsible for cell motility. Inspired by the rich disulfide exchange chemistry of integrins, we show here the inhibition of cell migration by cascade exchangers (CAXs), which also enable and inhibit cell penetration by thiol-mediated uptake. Fast-moving CAXs such as reversible Michael acceptor dimers, dithiabismepanes, and bioinspired epidithiodiketopiperazines are best, much better than Ellman's reagent. The implication that integrins participate in thiol-mediated uptake is confirmed by reduced uptake in integrin-knockdown cells. Although thiol-mediated uptake is increasingly emerging as a unifying pathway to bring matter into cells, its molecular basis is essentially unknown. These results identify the integrin superfamily as experimentally validated general cellular partners in the dynamic covalent exchange cascades that are likely to account for thiol-mediated uptake. The patterns identified testify to the complexity of the dynamic covalent networks involved. This work also provides chemistry tools to explore cell motility and expands the drug discovery potential of CAXs from antiviral toward antithrombotic and antitumor perspectives.

Comprehensive Analyses of Coagulation Parameters in Patients with Vascular Anomalies

BACKGROUND

Vascular anomalies comprise a diverse group of rare diseases with altered blood flow and are often associated with coagulation disorders. The most common example is a localized intravascular coagulopathy in venous malformations leading to elevated D-dimers. In severe cases, this may progress to a disseminated intravascular coagulopathy with subsequent consumption of fibrinogen and thrombocytes predisposing to serious bleeding. A separate coagulopathy is the Kasabach-Merritt phenomenon in kaposiform hemangioendothelioma characterized by platelet trapping leading to thrombocytopenia and eventually consumptive coagulopathy. Our previous work showed impaired von Willebrand factor and platelet aggregometry due to abnormal blood flow, i.e., in ventricular assist devices or extracorporeal membrane oxygenation. With altered blood flow also present in vascular anomalies, we hypothesized that, in particular, the von Willebrand factor parameters and the platelet function may be similarly impacted.

METHODS

We prospectively recruited 73 patients with different vascular anomaly entities and analyzed their coagulation parameters.

RESULTS

Acquired von Willebrand syndrome was observed in both of our patients with Kasabach-Merritt phenomenon. In six out of nine patients with complex lymphatic anomalies, both the vWF antigen and activity were upregulated. Platelet aggregometry was impaired in both patients with Kasabach-Merritt phenomenon and in seven out of eight patients with an arteriovenous malformation.

CONCLUSIONS

The analysis of coagulation parameters in our patients with vascular anomalies advanced our understanding of the underlying pathophysiologies of the observed coagulopathies. This may lead to new treatment options for the, in part, life-threatening bleeding risks in these patients in the future.

A Novel GATA1 Variant in the C-Terminal Zinc Finger Compared with the Platelet Phenotype of Patients with A Likely Pathogenic Variant in the N-Terminal Zinc Finger

The GATA1 transcription factor is essential for normal erythropoiesis and megakaryocytic differentiation. Germline GATA1 pathogenic variants in the N-terminal zinc finger (N-ZF) are typically associated with X-linked thrombocytopenia, platelet dysfunction, and dyserythropoietic anemia. A few variants in the C-terminal ZF (C-ZF) domain are described with normal platelet count but altered platelet function as the main characteristic. Independently performed molecular genetic analysis identified a novel hemizygous variant (c.865C>T, p.H289Y) in the C-ZF region of GATA1 in a German patient and in a Spanish patient. We characterized the bleeding and platelet phenotype of these patients and compared these findings with the parameters of two German siblings carrying the likely pathogenic variant p.D218N in the GATA1 N-ZF domain. The main difference was profound thrombocytopenia in the brothers carrying the p.D218N variant compared to a normal platelet count in patients carrying the p.H289Y variant; only the Spanish patient occasionally developed mild thrombocytopenia. A functional platelet defect affecting αIIbβ3 integrin activation and α-granule secretion was present in all patients. Additionally, mild anemia, anisocytosis, and poikilocytosis were observed in the patients with the C-ZF variant. Our data support the concept that GATA1 variants located in the different ZF regions can lead to clinically diverse manifestations.

Protein disulfide isomerase modulation of TRPV1 controls heat hyperalgesia in chronic pain

Protein disulfide isomerase (PDI) plays a key role in maintaining cellular homeostasis by mediating protein folding via catalyzing disulfide bond formation, breakage, and rearrangement in the endoplasmic reticulum. Increasing evidence suggests that PDI can be a potential treatment target for several diseases. However, the function of PDI in the peripheral sensory nervous system is unclear. Here we report the expression and secretion of PDI from primary sensory neurons is upregulated in inflammatory and neuropathic pain models. Deletion of PDI in nociceptive DRG neurons results in a reduction in inflammatory and neuropathic heat hyperalgesia. We demonstrate that secreted PDI activates TRPV1 channels through oxidative modification of extracellular cysteines of the channel, indicating that PDI acts as an unconventional positive modulator of TRPV1. These findings suggest that PDI in primary sensory neurons plays an important role in development of heat hyperalgesia and can be a potential therapeutic target for chronic pain.

PCSK9 Promotes Cardiovascular Diseases: Recent Evidence about Its Association with Platelet Activation-Induced Myocardial Infarction

Cardiovascular diseases are the leading cause of death worldwide, with the majority of the cases being heart failure due to myocardial infarction. Research on cardiovascular diseases is currently underway, particularly on atherosclerosis prevention, to reduce the risk of myocardial infarction. Proprotein convertase subtilisin/kexin type 9 (PCSK9) has been reported to play a role in lipid metabolism, by enhancing low-density lipoprotein (LDL) receptor degradation. Therefore, PCSK9 inhibitors have been developed and found to successfully decrease LDL plasma levels. Recent experimental studies have also implicated PCSK9 in platelet activation, having a key role during atherosclerosis progression. Although numerous studies have addressed the role of PCSK9 role in controlling hypercholesterolemia, studies and discussions exploring its involvement in platelet activation are still limited. Hence, here, we address our current understanding of the pathophysiological process involved in atherosclerosis-induced myocardial infarction (MI) through platelet activation and highlight the molecular mechanisms used by PCSK9 in regulating platelet activation. Undoubtedly, a deeper understanding of the relationship between platelet activation and the underlying molecular mechanisms of PCSK9 in the context of MI progression will provide a new strategy for developing drugs that selectively inhibit the most relevant pathways in cardiovascular disease progression.

Hermansky-Pudlak Syndrome: Identification of Novel Variants in the Genes HPS3, HPS5, and DTNBP1 (HPS-7)

Hermansky-Pudlak syndrome (HPS), a rare heterogeneous autosomal recessive disorder, is characterized by oculocutaneous albinism (OCA) and a bleeding diathesis due to a defect regarding melanosomes and platelet delta (δ)-granule secretion. Interestingly, patients with HPS type 2 (HPS-2) or HPS type 10 (HPS-10) present additionally with an immunological defect. We investigated three patients (IP1, IP2, and IP3) who suffer from a bleeding diathesis. Platelet aggregometry showed impaired platelet function and flow cytometry revealed a severely reduced platelet CD63 expression hinting to either a defect of platelet delta granule secretion or a decreased number of delta granules in these patients. However, only IP3 presents with an apparent OCA. We performed panel sequencing and identified a homozygous deletion of exon 6 in DTNBP1 for IP3. Western analysis confirmed the absence of the encoded protein dysbindin confirming the diagnosis of HPS-7. Interestingly, this patient reported additionally recurrent bacterial infections. Analysis of lymphocyte cytotoxicity showed a slightly reduced NK-degranulation previously documented in a more severe form in patients with HPS-2 or HPS-10. IP1 is carrier of two compound heterozygous variants in the HPS3 gene (c.65C > G and c.1193G > A). A homozygous variant in HPS5 (c.760G > T) was identified in IP2. The novel missense variants were classified as VUS (variant of uncertain significance) according to ACMG guidelines. For IP1 with the compound heterozygous variants in HPS3 a specialized ophthalmological examination showed ocular albinism. HPS3 and HPS5 encode subunits of the BLOC-2 complex and patients with HPS-3 or HPS-5 are known to present with variable/mild hypopigmentation.

ERO1-PDI Redox Signaling in Health and Disease

Significance: Protein disulfide isomerase (PDI) and endoplasmic reticulum oxidoreductase 1 (ERO1) are crucial for oxidative protein folding in the endoplasmic reticulum (ER). These enzymes are frequently overexpressed and secreted, and they contribute to the pathology of neurodegenerative, cardiovascular, and metabolic diseases. Recent Advances: Tissue-specific knockout mouse models and pharmacologic inhibitors have been developed to advance our understanding of the cell-specific functions of PDI and ERO1. In addition to their roles in protecting cells from the unfolded protein response and oxidative stress, recent studies have revealed that PDI and ERO1 also function outside of the cells. Critical Issues: Despite the well-known contributions of PDI and ERO1 to specific disease pathology, the detailed molecular and cellular mechanisms underlying these activities remain to be elucidated. Further, although PDI and ERO1 inhibitors have been identified, the results from previous studies require careful evaluation, as many of these agents are not selective and may have significant cytotoxicity. Future Directions: The functions of PDI and ERO1 in the ER have been extensively studied. Additional studies will be required to define their functions outside the ER.

A Novel Likely Pathogenic Variant in the BLOC1S5 Gene Associated with Hermansky-Pudlak Syndrome Type 11 and an Overview of Human BLOC-1 Deficiencies

Hermansky-Pudlak syndrome (HPS) is a heterogeneous disorder combining oculocutaneous albinism (OCA) and a platelet function disorder of varying severity as its most prominent features. The genes associated with HPS encode for different BLOC- (biogenesis of lysosome-related organelles complex) complexes and for the AP-3 (adaptor protein-3) complex, respectively. These proteins are involved in maturation, trafficking, and the function of lysosome-related organelles (LROs) such as melanosomes and platelet δ-granules. Some patients with different types of HPS can develop additional complications and symptoms like pulmonary fibrosis, granulomatous colitis, and immunodeficiency. A new type of HPS has recently been identified associated with genetic alterations in the BLOC1S5 gene, which encodes the subunit Muted of the BLOC-1 complex. Our aim was to unravel the genetic defect in two siblings with a suspected HPS diagnosis (because of OCA and bleeding symptoms) using next generation sequencing (NGS). Platelet functional analysis revealed reduced platelet aggregation after stimulation with ADP and a severe secretion defect in platelet δ-granules. NGS identified a novel homozygous essential splice site variant in the BLOC1S5 gene present in both affected siblings who are descendants of a consanguine marriage. The patients exhibited no additional symptoms. Our study confirms that pathogenic variants of BLOC1S5 cause the recently described HPS type 11.

Pathophysiological roles of cell surface and extracellular protein disulfide isomerase and their molecular mechanisms

Protein disulfide isomerase (PDI) is the prototypic member of the thiol isomerase family that catalyses disulfide bond rearrangement. Initially identified in the endoplasmic reticulum as folding catalysts, PDI and other members in its family have also been widely reported to reside on the cell surface and in the extracellular matrix. Although how PDI is exported and retained on the cell surface remains a subject of debate, this unique pool of PDI is developing into an important mechanism underlying the redox regulation of protein sulfhydryls that are critical for the cellular activities under various disease conditions. This review aims to provide an overview of the pathophysiological roles of surface and extracellular PDI and their underlying molecular mechanisms. Understanding the involvement of extracellular PDI in these diseases will advance our knowledge in the molecular aetiology to facilitate the development of novel pharmacological strategies by specifically targeting PDI in extracellular compartments.

Age-Dependent Control of Collagen-Dependent Platelet Responses by Thrombospondin-1-Comparative Analysis of Platelets from Neonates, Children, Adolescents, and Adults

Platelet function is developmentally regulated. Healthy neonates do not spontaneously bleed, but their platelets are hypo-reactive to several agonists. The mechanisms underlying immature platelet function in neonates are incompletely understood. This critical issue remains challenging for the establishment of age-specific reference ranges. In this study, we evaluated platelet reactivity of five pediatric age categories, ranging from healthy full-term neonates up to adolescents (11–18 years) in comparison to healthy adults (>18 years) by flow cytometry. We confirmed that platelet hypo-reactivity detected by fibrinogen binding, P-selectin, and CD63 surface expression was most pronounced in neonates compared to other pediatric age groups. However, maturation of platelet responsiveness varied with age, agonist, and activation marker. In contrast to TRAP and ADP, collagen-induced platelet activation was nearly absent in neonates. Granule secretion markedly remained impaired at least up to 10 years of age compared to adults. We show for the first time that neonatal platelets are deficient in thrombospondin-1, and exogenous platelet-derived thrombospondin-1 allows platelet responsiveness to collagen. Platelets from all pediatric age groups normally responded to the C-terminal thrombospondin-1 peptide RFYVVMWK. Thus, thrombospondin-1 deficiency of neonatal platelets might contribute to the relatively impaired response to collagen, and platelet-derived thrombospondin-1 may control distinct collagen-induced platelet responses.

Juglone prevents human platelet aggregation through inhibiting Akt and protein disulfide isomerase

BACKGROUND/PURPOSE

Juglone, a natural compound widely found in Juglandaceae plants, has been suggested as a potential drug candidate for treating cancer, inflammation, and diabetic vascular complications. In the present study, the antiplatelet effect and underlying mechanisms of juglone were investigated for the first time.

STUDY DESIGN/METHODS

Human platelet aggregation and activation were measured by turbidimetric aggregometry, flow cytometry, and Western blotting. In vitro antithrombotic activity of juglone was assessed using collagen-coated flow chambers under whole-blood flow conditions. The effect of juglone on protein disulfide isomerase (PDI) activity was determined by the dieosin glutathione disulfide assay.

RESULTS

Juglone (1 - 5 μM) inhibited platelet aggregation and glycoprotein (GP) IIb/IIIa activation caused by various agonists. In a whole blood flow chamber system, juglone reduced thrombus formation on collagen-coated surfaces under arterial shear rates. Juglone abolished intracellular Ca²⁺ elevation and protein kinase C activation caused by collagen, but had no significant effect on that induced by G protein-coupled receptor agonists. In contrast, Akt activation caused by various agonists were inhibited in juglone-treated platelets. Additionally, juglone showed inhibitory effects on both recombinant human PDI and platelet surface PDI at concentrations similar to those needed to prevent platelet aggregation.

CONCLUSION

Juglone exhibits potent in vitro antiplatelet and antithrombotic effects that are associated with inhibition of Akt activation and platelet surface PDI activity.

Zafirlukast is a broad-spectrum thiol isomerase inhibitor that inhibits thrombosis without altering bleeding times

BACKGROUND AND PURPOSE

Multiple members of the thiol isomerase (TI) family of enzymes are present in and released by platelets. Inhibition of these enzymes results in diminished platelet responses, aggregation, adhesion and thrombus formation. Recently, the therapeutic potential of TI inhibition has been recognised and drug-development technologies were used to identify selective small molecule inhibitors. To date, few pan-TI inhibitors have been characterised and the most studied, bacitracin, is known to be nephrotoxic, which prohibits its systemic therapeutic usage.

EXPERIMENTAL APPROACH

We therefore sought to identify novel broad-spectrum inhibitors of these enzymes and test their effects in vivo. A total of 3,641 compounds were screened for inhibitory effects on the redox activity of ERp5, protein disulphide isomerase (PDI), ERp57, ERp72 and thioredoxin in an insulin turbidity assay. Of the lead compounds identified, zafirlukast was selected for further investigation.

KEY RESULTS

When applied to platelets, zafirlukast diminished platelet responses in vitro. Zafirlukast was antithrombotic in murine models of thrombosis but did not impair responses in a model of haemostasis. Since TIs are known to modulate adhesion receptor function, we explored the effects of zafirlukast on cell migration. This was inhibited independently of cysteinyl LT receptor expression and was associated with modulation of cell-surface free thiol levels consistent with alterations in redox activity on the cell surface.

CONCLUSION AND IMPLICATIONS

We identify zafirlukast to be a novel, potent, broad-spectrum TI inhibitor, with wide-ranging effects on platelet function, thrombosis and integrin-mediated cell migration. Zafirlukast is antithrombotic but does not cause bleeding.

Plasmin-Induced Activation of Human Platelets Is Modulated by Thrombospondin-1, Bona Fide Misfolded Proteins and Thiol Isomerases

Inflammatory processes are triggered by the fibrinolytic enzyme plasmin. Tissue-type plasminogen activator, which cleaves plasminogen to plasmin, can be activated by the cross-β-structure of misfolded proteins. Misfolded protein aggregates also represent substrates for plasmin, promoting their degradation, and are potent platelet agonists. However, the regulation of plasmin-mediated platelet activation by misfolded proteins and vice versa is incompletely understood. In this study, we hypothesize that plasmin acts as potent agonist of human platelets in vitro after short-term incubation at room temperature, and that the response to thrombospondin-1 and the bona fide misfolded proteins Eap and SCN^--denatured IgG interfere with plasmin, thereby modulating platelet activation. Plasmin dose-dependently induced CD62P surface expression on, and binding of fibrinogen to, human platelets in the absence/presence of plasma and in citrated whole blood, as analyzed by flow cytometry. Thrombospondin-1 pre-incubated with plasmin enhanced these plasmin-induced platelet responses at low concentration and diminished them at higher dose. Platelet fibrinogen binding was dose-dependently induced by the C-terminal thrombospondin-1 peptide RFYVVMWK, Eap or NaSCN-treated IgG, but diminished in the presence of plasmin. Blocking enzymatically catalyzed thiol-isomerization decreased plasmin-induced platelet responses, suggesting that plasmin activates platelets in a thiol-dependent manner. Thrombospondin-1, depending on the concentration, may act as cofactor or inhibitor of plasmin-induced platelet activation, and plasmin blocks platelet activation induced by misfolded proteins and vice versa, which might be of clinical relevance.

Vascular thiol isomerases in thrombosis: The yin and yang

There has recently been considerable progress of the field of extracellular protein disulfide isomerases with vascular thiol isomerases in the forefront. Four members of protein disulfide isomerase (PDI) family of enzymes, PDI, ERp57, ERp72, and ERp5, have been shown to be secreted from activated platelets and endothelial cells at the site of vascular injury. Each isomerase individually supports platelet accumulation and coagulation, as indicated by multiple levels of evidence, including inhibitory antibodies, targeted knockout mice, and mutant isomerases. The transmembrane PDI family member TMX1 was recently shown to inhibit platelet function and thrombosis, demonstrating that the PDIs can have opposing functions in thrombosis. These observations provide a new concept that thiol isomerases can both positively and negatively regulate hemostasis, constituting off-on redox switches controlling activation of hemostatic factors. This redox network serves to maintain vascular homeostasis. Integrins such as the αIIbβ3 fibrinogen receptor on platelets appear to be major substrates, with the platelet receptor for von Willebrand factor, glycoprotein Ibα, as another substrate. S-nitrosylation of the prothrombotic PDIs may additionally negatively regulate platelets and thrombosis. Thiol isomerases also regulate coagulation in mouse models, and a clinical trial with the oral PDI inhibitor isoquercetin substantially decreased markers of coagulation in patients at risk for thrombosis. This review updates recent findings in the field and addresses emerging evidence that thiol/disulfide-based reactions mediated by the prothrombotic secreted PDIs are balanced by the transmembrane member of this family, TMX1.

Molecular Drivers of Platelet Activation: Unraveling Novel Targets for Anti-Thrombotic and Anti-Thrombo-Inflammatory Therapy

Cardiovascular diseases (CVDs) are the leading cause of death globally-partly a consequence of increased population size and ageing-and are major contributors to reduced quality of life. Platelets play a major role in hemostasis and thrombosis. While platelet activation and aggregation are essential for hemostasis at sites of vascular injury, uncontrolled platelet activation leads to pathological thrombus formation and provokes thrombosis leading to myocardial infarction or stroke. Platelet activation and thrombus formation is a multistage process with different signaling pathways involved to trigger platelet shape change, integrin activation, stable platelet adhesion, aggregation, and degranulation. Apart from thrombotic events, thrombo-inflammation contributes to organ damage and dysfunction in CVDs and is mediated by platelets and inflammatory cells. Therefore, in the past, many efforts have been made to investigate specific signaling pathways in platelets to identify innovative and promising approaches for novel antithrombotic and anti-thrombo-inflammatory strategies that do not interfere with hemostasis. In this review, we focus on some of the most recent data reported on different platelet receptors, including GPIb-vWF interactions, GPVI activation, platelet chemokine receptors, regulation of integrin signaling, and channel homeostasis of NMDAR and PANX1.

Protein Disulphide Isomerase A1 Is Involved in the Regulation of Breast Cancer Cell Adhesion and Transmigration via Lung Microvascular Endothelial Cells

Cancer cell cross-talk with the host endothelium plays a crucial role in metastasis, but the underlying mechanisms are still not fully understood. We studied the involvement of protein disulphide isomerase A1 (PDIA1) in human breast cancer cell (MCF-7 and MDA-MB-231) adhesion and transendothelial migration. For comparison, the role of PDIA1 in proliferation, migration, cell cycle and apoptosis was also assessed. Pharmacological inhibitor, bepristat 2a and PDIA1 silencing were used to inhibit PDIA1. Inhibition of PDIA1 by bepristat 2a markedly decreased the adhesion of breast cancer cells to collagen type I, fibronectin and human lung microvascular endothelial cells. Transendothelial migration of breast cancer cells across the endothelial monolayer was also inhibited by bepristat 2a, an effect not associated with changes in ICAM-1 expression or changes in cellular bioenergetics. The silencing of PDIA1 produced less pronounced anti-adhesive effects. However, inhibiting extracellular free thiols by non-penetrating blocker p-chloromercuribenzene sulphonate substantially inhibited adhesion. Using a proteomic approach, we identified that β1 and α2 integrins were the most abundant among all integrins in breast cancer cells as well as in lung microvascular endothelial cells, suggesting that integrins could represent a target for PDIA1. In conclusion, extracellular PDIA1 plays a major role in regulating the adhesion of cancer cells and their transendothelial migration, in addition to regulating cell cycle and caspase 3/7 activation by intracellular PDIA1. PDIA1-dependent regulation of cancer-endothelial cell interactions involves disulphide exchange and most likely integrin activation but is not mediated by the regulation of ICAM-1 expression or changes in cellular bioenergetics in breast cancer or endothelial cells.

Peri/Epicellular Thiol Oxidoreductases as Mediators of Extracellular Redox Signaling

Significance: Supracellular redox networks regulating cell-extracellular matrix (ECM) and organ system architecture merge with structural and functional (catalytic or allosteric) properties of disulfide bonds. This review addresses emerging evidence that exported thiol oxidoreductases (TORs), such as thioredoxin, protein disulfide isomerases (PDIs), quiescin sulfhydryl oxidases (QSOX)1, and peroxiredoxins, composing a peri/epicellular (pec)TOR pool, mediate relevant signaling. pecTOR functions depend mainly on kinetic and spatial regulation of thiol-disulfide exchange reactions governed by redox potentials, which are modulated by exported intracellular low-molecular-weight thiols, together conferring signal specificity. Recent Advances: pecTOR redox-modulates several targets including integrins, ECM proteins, surface molecules, and plasma components, although clear-cut documentation of direct effects is lacking in many cases. TOR catalytic pathways, displaying common patterns, culminate in substrate thiol reduction, oxidation, or isomerization. Peroxiredoxins act as redox/peroxide sensors, contrary to PDIs, which are likely substrate-targeted redox modulators. Emerging evidence suggests important pecTOR roles in patho(physio)logical processes, including blood coagulation, vascular remodeling, mechanosensing, endothelial function, immune responses, and inflammation. Critical Issues: Effects of pecPDIs supporting thrombosis/platelet activation have been well documented and reached the clinical arena. Roles of pecPDIA1 in vascular remodeling/mechanosensing are also emerging. Extracellular thioredoxin and pecPDIs redox-regulate immunoinflammation. Routes of TOR externalization remain elusive and appear to involve Golgi-independent routes. pecTORs are particularly accessible drug targets. Future Directions: Further understanding mechanisms of thiol redox reactions and developing assays for assessing pecTOR redox activities remain important research avenues. Also, addressing pecTORs as disease markers and achieving more efficient/specific drugs for pecTOR modulation are major perspectives for diagnostic/therapeutic improvements.

Protein disulfide isomerase in cardiovascular disease

Protein disulfide isomerase (PDI) participates in the pathogenesis of numerous diseases. Increasing evidence indicates that intravascular cell-derived PDI plays an important role in the initiation and progression of cardiovascular diseases, including thrombosis and vascular inflammation. Recent studies with PDI conditional knockout mice have advanced our understanding of the function of cell-specific PDI in disease processes. Furthermore, the identification and development of novel small-molecule PDI inhibitors has led into a new era of PDI research that transitioned from the bench to bedside. In this review, we will discuss recent findings on the regulatory role of PDI in cardiovascular disease.

Efforts to untangle the functions of a large family of enzymes could lead researchers to new therapies for diverse cardiovascular diseases. Members of the protein disulfide isomerase (PDI) family chemically modify other proteins in ways that can alter both their structure and biological activity. Jaehyung Cho of the University of Illinois at Chicago, USA and coworkers have reviewed numerous studies linking PDI with cardiovascular diseases, including thrombosis, heart attack, vascular inflammation, and stroke. The authors also report progress in developing small-molecule PDI inhibitors that could yield the treatment for these conditions.

Novel Antiplatelet Agents in Cardiovascular Disease

Antiplatelet therapy reduces atherothrombotic risk and has therefore become a cornerstone in the treatment of cardiovascular disease. Aspirin, adenosine diphosphate P2Y12 receptor antagonists, glycoprotein IIb/IIIa inhibitors, and the thrombin receptor blocker vorapaxar are effective antiplatelet agents but significantly increase the risk of bleeding. Moreover, atherothrombotic events still impair the prognosis of many patients with cardiovascular disease despite established antiplatelet therapy. Over the last years, advances in the understanding of thrombus formation and hemostasis led to the discovery of various new receptors and signaling pathways of platelet activation. As a consequence, many new antiplatelet agents with high antithrombotic efficacy and supposedly only moderate effects on regular hemostasis have been developed and yielded promising results in preclinical and early clinical studies. Although their long journey from animal studies to randomized clinical trials and finally administration in daily clinical routine has just begun, some of the new agents may in the future become meaningful additions to the pharmacological armamentarium in cardiovascular disease.

Urate hydroperoxide oxidizes endothelial cell surface protein disulfide isomerase-A1 and impairs adherence

BACKGROUND

Extracellular surface protein disulfide isomerase-A1 (PDI) is involved in platelet aggregation, thrombus formation and vascular remodeling. PDI performs redox exchange with client proteins and, hence, its oxidation by extracellular molecules might alter protein function and cell response. In this study, we investigated PDI oxidation by urate hydroperoxide, a newly-described oxidant that is generated through uric acid oxidation by peroxidases, with a putative role in vascular inflammation.

METHODS

Amino acids specificity and kinetics of PDI oxidation by urate hydroperoxide was evaluated by LC-MS/MS and by stopped-flow. Oxidation of cell surface PDI and other thiol-proteins from HUVECs was identified using impermeable alkylating reagents. Oxidation of intracellular GSH and GSSG was evaluated with specific LC-MS/MS techniques. Cell adherence, detachment and viability were assessed using crystal violet staining, cellular microscopy and LDH activity, respectively.

RESULTS

Urate hydroperoxide specifically oxidized cysteine residues from catalytic sites of recombinant PDI with a rate constant of 6 × 10³ M^-1 s^-1. Incubation of HUVECs with urate hydroperoxide led to oxidation of cell surface PDI and other unidentified cell surface thiol-proteins. Cell adherence to fibronectin coated plates was impaired by urate hydroperoxide, as well as by other oxidants, thiol alkylating agents and PDI inhibitors. Urate hydroperoxide did not affect cell viability but significantly decreased GSH/GSSG ratio.

CONCLUSIONS

Our results demonstrated that urate hydroperoxide affects thiol-oxidation of PDI and other cell surface proteins, impairing cellular adherence.

GENERAL SIGNIFICANCE

These findings could contribute to a better understanding of the mechanism by which uric acid affects endothelial cell function and vascular homeostasis.

Integrin-mediated cell adhesion requires extracellular disulfide exchange regulated by protein disulfide isomerase

Cell adhesion to extracellular matrix, mediated by integrin receptors, is crucial for cell survival. Receptor-ligand interaction involves conformational changes in the integrin by a mechanism not fully elucidated. In addition to several direct evidence that there is disulfide re-arrangement of integrins, we previously demonstrated a role for extracellular thiols and protein disulfide isomerase (PDI) in integrin-mediated functions using platelets as model system. Exploring the possible generality of this mechanism, we now show, using three different nucleated cells which depend on adhesion for survival, that non-penetrating blockers of free thiols inhibit α2β1 and α5β1 integrin-mediated adhesion and that disulfide exchange takes place in that process. Inhibiting extracellular PDI mimics thiol blocking. Transfection with WT or enzymatically inactive PDI increased their membrane expression and enhanced cell adhesion, suggesting that PDI level is a limiting factor and that the chaperone activity of the enzyme contributes to adhesion. Exogenously added PDI also enhanced adhesion, further supporting the limiting factor of the enzyme. These data indicate that: a) Dependence on ecto-sulfhydryls for integrin-mediated adhesion is not exclusive to the platelet; b) PDI is involved in integrin-mediated adhesion, catalyzing disulfide bond exchange; c) PDI enhances cell adhesion by both its oxidoreductase activity and as a chaperone.

Impaired human hematopoiesis due to a cryptic intronic GATA1 splicing mutation

Abdulhay et al. report that an intronic genetic variant alters GATA1 splicing and presents as a distinct form of dyserythropoietic anemia in two unrelated patients. Functional studies demonstrate that the novel GATA1 isoform lacks observable activity and leads to a decrease in wild-type GATA1 levels in affected individuals.

Studies of allelic variation underlying genetic blood disorders have provided important insights into human hematopoiesis. Most often, the identified pathogenic mutations result in loss-of-function or missense changes. However, assessing the pathogenicity of noncoding variants can be challenging. Here, we characterize two unrelated patients with a distinct presentation of dyserythropoietic anemia and other impairments in hematopoiesis associated with an intronic mutation in GATA1 that is 24 nucleotides upstream of the canonical splice acceptor site. Functional studies demonstrate that this single-nucleotide alteration leads to reduced canonical splicing and increased use of an alternative splice acceptor site that causes a partial intron retention event. The resultant altered GATA1 contains a five–amino acid insertion at the C-terminus of the C-terminal zinc finger and has no observable activity. Collectively, our results demonstrate how altered splicing of GATA1, which reduces levels of the normal form of this master transcription factor, can result in distinct changes in human hematopoiesis.

The transmembrane protein disulfide isomerase TMX1 negatively regulates platelet responses

Secreted platelet protein disulfide isomerases, PDI, ERp57, ERp5, and ERp72, have important roles as positive regulators of platelet function and thrombosis. Thioredoxin-related transmembrane protein 1 (TMX1) was the first described transmembrane member of the protein disulfide isomerase family of enzymes. Using a specific antibody, the recombinant extracellular domain of TMX1 (rTMX1) protein, a knockout mouse model, and a thiol-labeling approach, we examined the role of TMX1 in platelet function and thrombosis. Expression of TMX1 on the platelet surface increased with thrombin stimulation. The anti-TMX1 antibody increased platelet aggregation induced by convulxin and thrombin, as well as potentiated platelet ATP release. In contrast, rTMX1 inhibited platelet aggregation and ATP release. TMX1-deficient platelets had increased aggregation, ATP release, αIIbβ3 activation, and P-selectin expression, which were reversed by addition of rTMX1. TMX1-knockout mice had increased incorporation of platelets into a growing thrombus in an FeCl3-induced mesenteric arterial injury model, as well as shortened tail-bleeding times. rTMX1 oxidized thiols in the αIIbβ3 integrin and TMX1-deficient platelets had increased thiols in the β3 subunit of αIIbβ3, consistent with oxidase activity of rTMX1 against αIIbβ3. Thus, TMX1 is the first identified extracellular inhibitor of platelet function and the first disulfide isomerase that negatively regulates platelet function.

Hypoxia evokes increased PDI and PDIA6 expression in the infarcted myocardium of ex-germ-free and conventionally raised mice

The prototypic protein disulfide isomerase (PDI), encoded by the P4HB gene, has been described as a survival factor in ischemic cardiomyopathy. However, the role of protein disulfide isomerase associated 6 (PDIA6) under hypoxic conditions in the myocardium remains enigmatic, and it is unknown whether the gut microbiota influences the expression of PDI and PDIA6 under conditions of acute myocardial infarction. Here, we revealed that, in addition to the prototypic PDI, the PDI family member PDIA6, a regulator of the unfolded protein response, is upregulated in the mouse cardiomyocyte cell line HL-1 when cultured under hypoxia. In vivo, in the left anterior descending artery (LAD) ligation mouse model of acute myocardial infarction, similar to PDI, PDIA6 protein expression was enhanced in the infarcted area (LAD+) relative to uninfarcted sham tissue or the neighbouring area at risk (LAD–) of C57BL/6J mice. Interestingly, we found that ex-germ-free (ex-GF) mice subjected to the LAD ligation model for 24 h had a reduced ejection fraction compared with their conventionally raised (CONV-R) SPF controls. Furthermore, the LAD+ area in the infarcted heart of ex-GF mice showed reduced PDIA6 expression relative to CONV-R controls, suggesting that the presence of a gut microbiota enhanced LAD ligation-triggered PDIA6 expression. Collectively, our results demonstrate that PDIA6 is upregulated in cardiomyocytes as a consequence of hypoxia. In the LAD mouse model, PDIA6 was also increased in the infarcted area under in vivo conditions, but this increase was suppressed in ex-GF mice relative to CONV-R controls.

This article has an associated First Person interview with the first author of the paper.

Summary: We identified PDIA6 as a hypoxia-induced element of the unfolded protein response in cardiomyocytes and infarcted mouse hearts. PDIA6 expression and ejection fractions were reduced in infarcted ex-germ-free mouse hearts.

Peri/epicellular protein disulfide isomerase-A1 acts as an upstream organizer of cytoskeletal mechanoadaptation in vascular smooth muscle cells

Although redox processes closely interplay with mechanoresponses to control vascular remodeling, redox pathways coupling mechanostimulation to cellular cytoskeletal organization remain unclear. The peri/epicellular pool of protein disulfide isomerase-A1 (pecPDIA1) supports postinjury vessel remodeling. Using distinct models, we investigated whether pecPDIA1 could work as a redox-dependent organizer of cytoskeletal mechanoresponses. In vascular smooth muscle cells (VSMCs), pecPDIA1 immunoneutralization impaired stress fiber assembly in response to equibiaxial stretch and, under uniaxial stretch, significantly perturbed cell repositioning perpendicularly to stretch orientation. During cyclic stretch, pecPDIA1 supported thiol oxidation of the known mechanosensor β₁-integrin and promoted polarized compartmentalization of sulfenylated proteins. Using traction force microscopy, we showed that pecPDIA1 organizes intracellular force distribution. The net contractile moment ratio of platelet-derived growth factor-exposed to basal VSMCs decreased from 0.90 ± 0.09 (IgG-exposed controls) to 0.70 ± 0.08 after pecPDI neutralization ( P < 0.05), together with an enhanced coefficient of variation for distribution of force modules, suggesting increased noise. Moreover, in a single cell model, pecPDIA1 neutralization impaired migration persistence without affecting total distance or velocity, whereas siRNA-mediated total PDIA1 silencing disabled all such variables of VSMC migration. Neither expression nor total activity of the master mechanotransmitter/regulator RhoA was affected by pecPDIA1 neutralization. However, cyclic stretch-induced focal distribution of membrane-bound RhoA was disrupted by pecPDI inhibition, which promoted a nonpolarized pattern of RhoA/caveolin-3 cluster colocalization. Accordingly, FRET biosensors showed that pecPDIA1 supports localized RhoA activity at cell protrusions versus perinuclear regions. Thus, pecPDI acts as a thiol redox-dependent organizer and noise reducer mechanism of cytoskeletal repositioning, oxidant generation, and localized RhoA activation during a variety of VSMC mechanoresponses. NEW & NOTEWORTHY Effects of a peri/epicellular pool of protein disulfide isomerase-A1 (pecPDIA1) during mechanoregulation in vascular smooth muscle cells (VSMCs) were highlighted using approaches such as equibiaxial and uniaxial stretch, random single cell migration, and traction force microscopy. pecPDIA1 regulates organization of the cytoskeleton and minimizes the noise of cell alignment, migration directionality, and persistence. pecPDIA1 mechanisms involve redox control of β₁-integrin and localized RhoA activation. pecPDIA1 acts as a novel organizer of mechanoadaptation responses in VSMCs.

Hereditary neuralgic amyotrophy in childhood caused by duplication within the SEPT9 gene: A family study

Hereditary neuralgic amyotrophy (HNA) is an autosomal dominant disorder associated with episodic, recurrent, and painful neuropathies affecting the nerves of the brachial plexus. In this study, we report on a family of Lebanese descent with HNA onset in early childhood. The affected family members presented with platelet dysfunction. Platelet aggregation was reduced after stimulation with the agonists ADP and epinephrine in all affected family members. Flow cytometric analyses revealed impaired platelet δ‐secretion. The index patient and one brother suffered from kidney cysts. Molecular genetic analysis revealed a heterozygous duplication of exon 2 within the septin 9 (SEPT9) gene in all the affected family members. Such a young child with HNA (aged 2 years) caused by SEPT9 duplication has not been described so far.

More than a syllable in fib-ROS-is: The role of ROS on the fibrotic extracellular matrix and on cellular contacts

Fibrosis is characterized by excess deposition of extracellular matrix (ECM). However, the ECM changes during fibrosis not only quantitatively but also qualitatively. Thus, the composition is altered as the expression of various ECM proteins changes. Moreover, also posttranslational modifications, secretion, deposition and crosslinkage as well as the proteolytic degradation of ECM components run differently during fibrosis. As several of these processes involve redox reactions and some of them are even redox-regulated, reactive oxygen species (ROS) influence fibrotic diseases. Redox regulation of the ECM has not been studied intensively, although evidences exist that the alteration of the ECM, including the redox-relevant processes of its formation and degradation, may be of key importance not only as a cause but also as a consequence of fibrotic diseases. Myofibroblasts, which have differentiated from fibroblasts during fibrosis, produce most of the ECM components and in return obtain important environmental cues of the ECM, including their redox-dependent fibrotic alterations. Thus, myofibroblast differentiation and fibrotic changes of the ECM are interdependent processes and linked with each other via cell-matrix contacts, which are mediated by integrins and other cell adhesion molecules. These cell-matrix contacts are also regulated by redox processes and by ROS. However, most of the redox-catalyzing enzymes are localized within cells. Little is known about redox-regulating enzymes, especially the ones that control the formation and cleavage of redox-sensitive disulfide bridges within the extracellular space. They are also important players in the redox-regulative crosstalk between ECM and cells during fibrosis.

Regulatory role of thiol isomerases in thrombus formation

INTRODUCTION

The protein disulfide isomerase (PDI) family of thiol isomerases are intracellular enzymes known to catalyze the oxidation, reduction and isomerization of disulfide bonds during protein synthesis in the endoplasmic reticulum. PDI and related members of the thiol isomerase family are known to localize extracellularly where they possess various functions. Among these, the role of PDI in the initiation of thrombus formation is best characterized. PDI is secreted within seconds from activated platelets and endothelial cells at the site of vascular injury and accumulates in the developing platelet-fibrin thrombus. Inhibition of PDI by antibodies or small molecule inhibitors blocks thrombus formation. Efforts are underway to identify extracellular substrates of PDI that participate in the network pathways linking thiol isomerases to thrombus formation. ERp57, ERp5 and ERp72 also play a role in initiation of thrombus formation but their specific extracellular substrates are unknown. Areas covered: The following review gives an overview of biochemistry of vascular thiol isomerases followed by a detailed description of their role in thrombosis and its clinical implications. Expert commentary: The thiol isomerase system, by controlling the initiation of thrombus formation, provides the regulatory switch by which the normal vasculature is protected under physiologic conditions from thrombi generation.

Acquired von Willebrand syndrome and impaired platelet function during venovenous extracorporeal membrane oxygenation: Rapid onset and fast recovery

BACKGROUND

Bleeding contributes to the high mortality of venovenous extracorporeal membrane oxygenation (vvECMO). The development of acquired von Willebrand syndrome (AVWS) has been identified as relevant pathology during ECMO. This study was performed to determine the onset of AVWS after implantation and the recovery of von Willebrand factor (VWF) parameters after explantation of ECMO in a large cohort of patients.

METHODS

VWF parameters of 59 patients treated with vvECMO at a university ECMO center were obtained before ECMO implantation, during therapy, and after explantation. In a subgroup of patients, light transmission aggregometry of platelets and flow-cytometric quantification of platelet granule secretion were performed.

RESULTS

All patients developed severe AVWS hours after implantation of vvECMO. After explantation, AVWS recovered within 3 hours in 60%, within 6 hours in 86%, and in all patients within 1 day. Aggregometry showed hypoaggregability of platelets after stimulation with ADP, ristocetin, collagen, and epinephrine. Flow-cytometric platelet analyses revealed severely reduced expression of CD62 and CD63.

CONCLUSIONS

All patients during vvECMO support rapidly develop AVWS and platelet dysfunction, resulting in severe impairment of coagulation. After explantation, AVWS overwhelmingly recovers within hours, resulting in a hypercoagulative state. These findings augment the need for novel extracorporeal technologies with reduced shear stress, and shift the emphasis for intense anti-coagulation during ECMO instead to a time-point after explantation.

Endothelial cells microparticle-associated protein disulfide isomerase promotes platelet activation in metabolic syndrome

BACKGROUND

Metabolic syndrome (MetS) is a common challenge in the world, and the platelet activation is enhanced in MetS patients. However, the fundamental mechanism that underlies platelet activation in MetS remains incompletely understood. Endothelial cells are damaged seriously in MetS patients, then they release more endothelial microparticles (EMPs). After all, whether the EMPs participate in platelet activation is still obscure. If they were, how did they work?

RESULTS

We demonstrated that the levels of EMPs, PMPs (platelet derived microparticles) and microparticle-carried-PDI activity increased in MetS patients. IR endothelial cells released more EMPs, the EMP-PDI was more activated. EMPs can enhance the activation of CD62P, GPIIb/IIIa and platelet aggregation and this process can be partly inhibited by PDI inhibitor such as RL90 and rutin. Activated platelets stimulated by EMPs expressed more PDI on cytoplasm and released more PMPs.

MATERIALS AND METHODS

We obtained plasma from 23 MetS patients and 8 normal healthy controls. First we built insulin resistance (IR) model of human umbilical vein endothelial cells (HUVECs), and then we separated EMPs from HUVECs culture medium and used these EMPs to stimulate platelets. Levels of microparticles, P-selectin(CD62P), Glycoprotein IIb/IIIa (GPIIb/IIIa) were detected by flow cytometry and levels of EMPs were detected by enzyme-linked immunosorbent assay (ELISA). The protein disulfide isomerase (PDI) activity was detected by insulin transhydrogenase assay. Platelet aggregation was assessed by turbidimetry.

CONCLUSION

EMPs can promote the activation of GPIIb/IIIa in platelets and platelet aggregation by the PDI which is carried on the surface of EMPs.

A humanized monoclonal antibody that inhibits platelet-surface ERp72 reveals a role for ERp72 in thrombosis

Essentials ERp72 is a thiol isomerase enzyme. ERp72 levels increase at the platelet surface during platelet activation. We generated a humanized monoclonal antibody which blocks ERp72 enzyme activity (anti-ERp72). Anti-ERp72 inhibits platelet functional responses and thrombosis.

SUMMARY

Background Within the endoplasmic reticulum, thiol isomerase enzymes modulate the formation and rearrangement of disulfide bonds in newly folded proteins entering the secretory pathway to ensure correct protein folding. In addition to their intracellular importance, thiol isomerases have been recently identified to be present on the surface of a number of cell types where they are important for cell function. Several thiol isomerases are known to be present on the resting platelet surface, including PDI, ERp5 and ERp57, and levels are increased following platelet activation. Inhibition of the catalytic activity of these enzymes results in diminished platelet function and thrombosis. Aim We previously determined that ERp72 is present at the resting platelet surface and levels increase upon platelet activation; however, its functional role on the cell surface was unclear. We aimed to investigate the role of ERp72 in platelet function and its role in thrombosis. Methods Using HuCAL technology, fully humanized Fc-null anti-ERp72 antibodies were generated. Eleven antibodies were screened for their ability to inhibit ERp72 activity and the most potent inhibitory antibody (anti-ERp72) selected for further testing in platelet functional assays. Results and conclusions Anti-ERp72 inhibited platelet aggregation, granule secretion, calcium mobilisation and integrin activation, revealing an important role for extracellular ERp72 in the regulation of platelet activation. Consistent with this, infusion of anti-ERp72 into mice protected against thrombosis.

Platelet Secretion Defects and Acquired von Willebrand Syndrome in Patients With Ventricular Assist Devices

Background

The number of implanted ventricular assist devices (VADs) has increased significantly recently. Bleeding, the most frequent complication, cannot be solely attributed to anticoagulation therapy. Acquired von Willebrand syndrome (AVWS) caused by increased shear stress is frequent in VAD patients and can increase the bleeding risk. The HeartMate III (HM III) is a novel left VAD featuring potential improvements over the HeartMate II.

Methods and Results

In this study, we investigated the prevalence and onset of AVWS in 198 VAD patients. To our knowledge, this is the largest cohort of VAD patients whose longitudinal data on AVWS have been collected. We also analyzed whether AVWS is less severe in HM III patients than in HeartMate II patients. Because platelet dysfunction can raise the bleeding risk, we investigated platelet function in a subset of patients. In total, 198 VAD patients and 60 patients with heart transplants as controls were included in this study. The ratio of von Willebrand factor collagen binding capacity to von Willebrand factor:antigen, multimer analyses, and platelet function (especially secretion of α‐ and δ‐granules) were investigated. All 198 VAD patients developed AVWS. As soon as the VAD was explanted, the AVWS disappeared within hours. AVWS was less severe in the HM III patients than in the HeartMate II patients. The HM III patients had fewer bleeding symptoms. In addition, VAD patients exhibited a platelet α‐ and δ‐granule secretion defect.

Conclusions

AVWS develops in VAD patients and may increase the bleeding risk. The HM III device causes less severe AVWS. Platelet secretion defects should be investigated in VAD patients because they also raise the bleeding risk.

Clinical Trial Registration

https://www.drks.de/drks_web. Unique identifier: DRKS00000649.

Components in Plasma-Derived Factor VIII, But Not in Recombinant Factor VIII Downregulate Anti-Inflammatory Surface Marker CD163 in Human Macrophages through Release of CXCL4 (Platelet Factor 4)

BACKGROUND

Hemarthrosis, or bleeding into the joints, is a hallmark of hemophilia. Heme triggers oxidative stress, inflammation, and destruction of cartilage and bone. The haptoglobin-CD163-heme oxygenase-1 (HO-1) pathway circumvents heme toxicity through enzymatic degradation of heme and transcription of antioxidant genes. Plasma-derived factor concentrates contain many proteins that might impact on cellular pathways in joints, blood, and vessels.

METHODS

Activation of platelets from healthy volunteers was assessed by flow cytometry analysis of fibrinogen binding and CD62P expression. Platelet CXCL4 release was measured by ELISA. Human peripheral blood mononuclear cells were exposed to CXCL4 or platelet supernatants (untreated or pre-stimulated with factor VIII (FVIII) products) during their differentiation to macrophages and analyzed for CD163 expression. Some macrophage cultures were additionally incubated with autologous hemoglobin for 18 h for analysis of HO-1 expression.

RESULTS

Platelet CXCL4 release was increased by all 8 tested plasma-derived FVIII products but not the 3 recombinant products. Macrophages exposed to supernatant from platelets treated with some plasma-derived FVIII products downregulated CD163 surface expression and failed to upregulate the athero- and joint protective enzyme HO-1 in response to hemoglobin.

CONCLUSION

Plasma-derived FVIII products might promote bleeding-induced joint injury via generation of macrophages that are unable to counteract redox stress.

Multifunctional molecule ERp57: From cancer to neurodegenerative diseases

The protein disulfide isomerase (PDI) gene family is a protein family classically characterized by endoplasmic reticulum (ER) localization and isomerase and redox activity. ERp57, a prominent multifunctional member of the PDI family, is detected at various levels in multiple cellular localizations outside of the ER. ERp57 has been functionally linked to a host of physiological processes and numerous studies have demonstrated altered expression and aberrant functionality of ERp57 in association with diverse pathological states. Here, we summarize available knowledge of ERp57's functions in subcellular compartments and the roles of dysregulated ERp57 in various diseases toward an emphasis on the potential utility of therapeutic development of ERp57.

HPW-RX40 prevents human platelet activation by attenuating cell surface protein disulfide isomerases

Protein disulfide isomerase (PDI) present at platelet surfaces has been considered to play an important role in the conformational change and activation of the integrin glycoprotein IIb/IIIa (GPIIb/IIIa) and thus enhances platelet aggregation. Growing evidences indicated that platelet surface PDI may serve as a potential target for developing of a new class of antithrombotic agents. In the present study, we investigated the effects of HPW-RX40, a chemical derivative of β-nitrostyrene, on platelet activation and PDI activity. HPW-RX40 inhibited platelet aggregation, GPIIb/IIIa activation, and P-selectin expression in human platelets. Moreover, HPW-RX40 reduced thrombus formation in human whole blood under flow conditions, and protects mice from FeCl₃-induced carotid artery occlusion. HPW-RX40 inhibited the activity of recombinant PDI family proteins (PDI, ERp57, and ERp5) as well as suppressed cell surface PDI activity of platelets in a reversible manner. Exogenous addition of PDI attenuated the inhibitory effect of HPW-RX40 on GPIIb/IIIa activation. Structure-based molecular docking simulations indicated that HPW-RX40 binds to the active site of PDI by forming hydrogen bonds. In addition, HPW-RX40 neither affected the cell viability nor induced endoplasmic reticulum stress in human cancer A549 and MDA-MB-231 cells. Taken together, our results suggest that HPW-RX40 is a reversible and non-cytotoxic PDI inhibitor with antiplatelet effects, and it may have a potential for development of novel antithrombotic agents.

Novel antiplatelet role for a protein disulfide isomerase-targeted peptide: evidence of covalent binding to the C-terminal CGHC redox motif

Essentials Inhibitors of protein disulfide isomerase (PDI) have been considered a new antithrombotic class. CxxC is a PDI-targeted peptide that has been previously shown to inhibit its reductase activity. CxxC binds to surface PDI and inhibits ADP- and thrombin-evoked platelet activation and aggregation. CxxC binds to Cys400 on CGHC redox motif of PDI a' domain, a site for PDI prothrombotic activity.

SUMMARY

Background Protein disulfide isomerase (PDI) plays a major role in platelet aggregation, and its inhibitors have emerged as novel antithrombotic drugs. In previous work, we designed a peptide based on a PDI redox motif (CGHC) that inhibited both PDI reductase activity and PDI-modulated superoxide generation by neutrophil Nox2. Thus, we hypothesized that this peptide would also inhibit platelet aggregation by association with surface PDI. Methods Three peptides were used: CxxC, containing the PDI redox motif; Scr, presenting a scrambled sequence of the same residues and AxxA, with cysteines replaced by alanine. These peptides were tested under platelet aggregation and flow cytometry protocols to identify their possible antiplatelet activity. We labeled membrane free thiol and electrospray ionization liquid chromatography tandem mass spectrometry to test for an interaction. Results CxxC decreased platelet aggregation in a dose-dependent manner, being more potent at lower agonist concentrations, whereas neither AxxA nor Scr peptides exerted any effect. CxxC decreased aIIbb3 activation, but had no effect on the other markers. CxxC also decreased cell surface PDI pulldown without interfering with the total thiol protein content. Finally, we detected the addition of one CxxC molecule to reduced PDI through binding to Cys400 through mass spectrometry. Interestingly, CxxC did not react with oxidized PDI. Discussion CxxC has consistently shown its antiplatelet effects, both in PRP and washed platelets, corroborated by decreased aIIbb3 activation. The probable mechanism of action is through a mixed dissulphide bond with Cys400 of PDI, which has been shown to be essential for PDI's actions. Conclusion In summary, our data support antiplatelet activity for CxxC through binding to Cys400 in the PDI a0 domain, which can be further exploited as a model for sitedriven antithrombotic agent development.

Distinct contributions of complement factors to platelet activation and fibrin formation in venous thrombus development

Expanding evidence indicates multiple interactions between the hemostatic system and innate immunity, and the coagulation and complement cascades. Here we show in a tissue factor (TF)-dependent model of flow restriction-induced venous thrombosis that complement factors make distinct contributions to platelet activation and fibrin deposition. Complement factor 3 (C3) deficiency causes prolonged bleeding, reduced thrombus incidence, thrombus size, fibrin and platelet deposition in the ligated inferior vena cava, and diminished platelet activation in vitro. Initial fibrin deposition at the vessel wall over 6 hours in this model was dependent on protein disulfide isomerase (PDI) and TF expression by myeloid cells, but did not require neutrophil extracellular trap formation involving peptidyl arginine deiminase 4. In contrast to C3^-/- mice, C5-deficient mice had no apparent defect in platelet activation in vitro, and vessel wall platelet deposition and initial hemostasis in vivo. However, fibrin formation, the exposure of negatively charged phosphatidylserine (PS) on adherent leukocytes, and clot burden after 48 hours were significantly reduced in C5^-/- mice compared with wild-type controls. These results delineate that C3 plays specific roles in platelet activation independent of formation of the terminal complement complex and provide in vivo evidence for contributions of complement-dependent membrane perturbations to prothrombotic TF activation on myeloid cells.

Protein disulfide isomerase externalization in endothelial cells follows classical and unconventional routes

Extracellular protein disulfide isomerase (PDIA1) pool mediates thrombosis and vascular remodeling, however its externalization mechanisms remain unclear. We performed systematic pharmacological screening of secretory pathways affecting extracellular PDIA1 in endothelial cells (EC). We identified cell-surface (csPDIA1) and secreted non-particulated PDIA1 pools in EC. Such Golgi bypass also occurred for secreted PDIA1 in EC at baseline or after PMA, thrombin or ATP stimulation. Inhibitors of Type I, II and III unconventional routes, secretory lysosomes and recycling endosomes, including syntaxin-12 deletion, did not impair EC PDIA1 externalization. This suggests predominantly Golgi-independent unconventional secretory route(s), which were GRASP55-independent. Also, these data reinforce a vesicular-type traffic for PDIA1. We further showed that PDIA1 traffic is ATP-independent, while actin or tubulin cytoskeletal disruption markedly increased EC PDIA1 secretion. Clathrin inhibition enhanced extracellular soluble PDIA1, suggesting dynamic cycling. Externalized PDIA1 represents <2% of intracellular PDIA1. PDIA1 was robustly secreted by physiological levels of arterial laminar shear in EC and supported alpha 5 integrin thiol oxidation. Such results help clarify signaling and homeostatic mechanisms involved in multiple (patho)physiological extracellular PDIA1 functions.